Tobacco chemistry, 48: Synthesis of tobacco labdonoids by sensitized photo-oxygenation of (12E)-abienol [Nicotiana tabacum]
1979
Wahlberg, I. | Nordfors, K. | Curvall, M. | Nishida, T. | Enzell, C.R.
The sensitized photo-oxygenation of (12E)-abienol (1) occurs with competition between four processes, the two major ones being syn ene reactions, which yield, after reduction, the (12S)- and (12R)-13(16),14-labdadiene-8,12-diols (2,3; 51 per cent) and the (11E,13S)- and (11E,13R)-11,14-labdadiene-8,13-diols (4,5; 23 per cent). Smaller contributions to the overall yield are obtained from a process which is proposed to involve the attack of oxygen on the 12,13 double bond and to proceed with participation of the 8alpha-hydroxyl group to the (12R,13S)- and (12S,13R)-8,12-epoxy-14-labden-13-ols (6, 7; 2 per cent) and the (12R,13S)- and (12S,13R)-8,13-epoxy-14-labden-12-ols (8,9), and from a Diels-Alder reaction (7 per cent) giving inter alia (12R)-12,15-epidioxy-13-labden-8-ol (10). Subsequent chemical alterations of the initially produced compounds account for the generation of the (12S)- and (12R)-15,16- epidioxy-13-labdene-8,12-diols (13,14), the (12S)-and (12R)-15,16-epoxy-13(16),14-labdadiene-8,12-diols (15, 16), (11E)-14,15-bisnor-8-hydroxy-11-labden-13-one (17) and 12-norambreinolide (18). Since eight of the products obtained are tobacco constituents, i.e. 4-9, 17 and 18, and since only four of these, 4, 5, 17 and 18, are formed on photo-oxygenation of (12Z)-abienol (19), it seems likely that both isomers of abienol (1, 19) are precursors of the labdanoids found in tobacco.
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